Moving material drying method and apparatus



July 1966 J. w. POWISCHILL MOVING MATERIAL DRYING METHOD AND APPARATUS Filed Feb. 5, 1964 4SheetS-Sheet 1 -11 i llr. a

IIIIIIIII lllIll INVENTOR: JOHN W. POWISCHI LL ATT VS y 1966 J. w. POWISCHILL MOVING MATERIAL DRYING METHOD AND APPARATUS Filed Feb. 5, 1964 4 Sheets-Sheet 2 JOHN W. POWISCI-IILL July 12, 1966 J. w. POWISCHILL 3,

MOVING MATERIAL DRYING METHOD AND APPARATUS Filed Feb. 5, 1964 4 Sheets-Sheet 5 TE MPERA 77/ E JOHN W. POWlSC ILL ATTYS y 12, 1966 J. w. POWISCHILL 3,259,995

MOVING MATERIAL DRYING METHOD AND APPARATUS Filed Feb. 5, 1964 4 Sheets-Sheet 4 FIG7.

TEMPE/YA TURE INVENTORI JOHN W. POWISCHILL ATTYS.

United States Patent 3,259,995 MOVING MATERIAL DRYING METHOD AND APPARATUS John W. Powischill, Cheltenham, Pa., assignor to Proctor This invention relates to method and apparatus for drying fibers, chemicals and other materials. More particularly the present invention relates to method and apparatus for automatically controlling drying conditions during processing of the material to compensate for initial variations in the moisture content of some of the material to produce a processed material all of which is characterized by a substantially uniform moisture content.

In some conventional drying apparatus for drying, for example, fibrous material, the material to be dried is continuously deposited in a layer at one end of a moving conveyor which transports the material through a drying chamber and during movement through the drying chamher, a drying medium is circulated through the layer of material on the conveyor to dry the same. At present, the final condition of the material is determined more or less by a trail and error basis, that is, during the initial stages of operation samples of the processed material are examined for moisture content and the speed of the conveyor, temperature, velocity and humidity of the drying medium are adjusted until the conditions are established to give the desired final moisture content. During operation, changes may be made as indicated by sampling or the operators experience.

For example, if the first sampling indicates the material is too Wet, a manual adjustment is made to slow down the conveyor to increase the drying time, and/or the temperature of the drying medium is raised or the velocity of the drying medium or the amount of exhaust and/-or the amount of fresh air is increased to increase the drying rate. On the other hand, if the first sampling indicates the processed material is too dry, the conditions in the dryer are adjusted to decrease the drying rate by increasing the conveyor speed and/or changing the condition or rate of flow of the drying medium, or reducing the amount of exhaust and fresh air. When the desired drying conditions, as determined above, are reached, the conditions are normally maintained fixed for processing all of the material, or manually adjusted from time to time to compensate for variations in the material. Adjustment of drying conditions by the operator from time to time as indicated by examining processed samples is not satisfactory for the reason that the sampling might indicate a decrease in the drying rate when in fact the material passing through the apparatus might require an increase in the drying rate or vice versa.

It has been found that any continuous run of material prior to drying is usually not characterized by a uniform moisture content due to variations in the inherent characteristics of different portions of the material and/or variations resulting from a preceding process such as treatment in squeeze rolls, filters or other dewatering means. Thus, it can be appreciated that material having a variable moisture contentwhich is dried under predetermined, fixed conditions also has a variable moisture content after processing.

With the foregoing in mind, an object of the present invention is to provide a method and apparatus for continuously drying material wherein drying conditions are automatically regulated in response to variations in moisture content of some of the material being dried whereby all the material processed is characterized by a substantially uniform moisture content.

Another object of the present invention is to provide a process for continuously drying material to provide a finished product characterized by a uniform moisture content and apparatus for carrying out the method which is of comparatively simplified construction and may be manufactured economically.

These and other objects of the present invention and the various features and details of the operation of the apparatus and process and construction of the apparatus are hereinafter more fully set forth with reference to the accompanying drawings, in which:

FIG. 1 is a side elevational view of a dryer in accordance with the present invention;

FIG. 2 is an enlarged sectional View taken on line 2-2 of FIG. 1;

FIG. 3 is a side elevational view of another embodiment of dryer in accordance with the present invention;

FIG. 3a is an enlarged sectional view taken on line 3a3a of FIG. 3;

FIG. 4 is a schematic showing of the moisture content control system for the dryer shown in FIGS. 1 and 2;

FIG. 5 is a schematic showing of the moisture content control system for the dryer shown in FIG. 3;

FIG. 6 is' a graph wherein the vertical axis represents the temperature of the drying medium and the horizontal axis represents the distance of the material from the starting point; and

FIG. 7 is a graph similar to FIG. 6 showing a drying curve for a material having little or no free moisture.

In the course of drying material having a predetermined initial moisture content wherein the condition, flow rate and distribution of the drying medium are maintained constant, the temperature drop of the drying medium across the material follows a definite pattern as shown in the chart of FIG. 6. As illustrated, the drying process can 'be divided into a constant rate drying period P wherein the temperature drop of the drying medium is substantially constant and a falling rate drying period P wherein the temperature drop of the drying medium across the material gradually decreases approximately linearly. In the chart, the solid line D represents the temperature of the drying medium prior to contacting the material to be dried and the line D represents the temperature of the drying medium after passing through the material. The point at which the constant rate drying period P terminates and the falling rate drying period P, begins is known as the critical point C.

During the constant rate drying period P the material loses water at a constant rate, the water evaporated during this period being for the most part that which exists on the surface of the material known as free or unbound moisture. During the falling rate drying period Pf, the bound or hygroscopic moisture intimately asso ciated with the physical nature of the material is removed and as noted above, the rate at which moisture is removed progressively decreases.

For a given quantity of material having a uniform initial moisture content throughout, it is possible to set the drying conditions so that the constant rate drying period P terminates after a predetermined time T in the drying cycle and the temperature after passage through the material follows the line D to provide a dried material having a desired moisture content after a given length of time T in the dryer.

In most instances, variations in the moisture content of some portions of the material are in the free or unbound moisture. Accordingly, when this is the case, the time of the constant rate drying period P extends or shortens relative to the given time T,, depending on Whether the free moisture is in excess of or less than a predetermined amount, and the critical point shifts to the right or left of the desired critical point C. In general, the temperature drop line in the falling rate drying period P: parallels the desired line D, and the material will be either too wet or too dry after the given time T. For example, if the material being processed has a lesser amount of initial moisture than a predetermined amount, the time to reach the critical moisture content is reduced and the critical point C on the curve moves to the left to point C (FIG. 6). Conversely, if the initial moisture content is higher than the predetermined amount, it will require a longer time during the constant rate period to reach the critical point and the point will move to the right to point C The falling rate portion in any case should then occur at approximately the same rate so a parallel family of curves result when plotting temperatures versus time, with temperature measured after the drying medium has passed through the conveyor.

In accordance with the present invention, a method is provided for controlling the drying of material whereby the final moisture content of the material may be more accurately controlled irrespective of variations in the moisture content in a particular portion or'portions of the material being processed. In the present instance, moisture content control is accomplished by continuously sensing (at a scanning point S in the falling rate drying period P the temperature of the drying medium after passage through the material relative to a predetermined theoretical temperature on the line D at the scanning point and then automatically varying the drying conditions and/or drying rate in response to deviations in the temperature drop of the drying medium relative to a predetermined temperature drop at the scanning point. By this method, variations in the initial moisture content of some portions of an entire batch of material are compensated for continuously and all of the processed material is characterized by a substantially uniform moisture content. In certain materials there is not a clear distinction between the rate of drying during the constant rate "period P and the falling rate period P If there is little or no free moisture in the material being processed, there may not be a constant rate period at all and the temperature drop of the drying medium will be continuously decreasing. Also, the falling rate period may be characterized by a non-linear curve rather than a substantially linear curve as previously described. In such cases the critical moisture content is not exactly determined, but can only be approximated. A time-temperature curve for such a condition is shown in FIG. 7. The point at which the two extended lines meet as at C may be considered as equivalent to the critical point.

When materials are of such a nature that there is no constant rate period at all, the method of sensing and correcting described may still be used, but it then becomes necessary to determine by experiment and test the exact temperature drop required at the sensing point so as to correlate this to some exact moisture content. In this case a variation from this exact temperature drop will indicate a variation in moisture at this point, i.e., a higher temperature drop means a higher moisture content, a lower temperature drop, a lower moistu r'e content, and again corrections can be made based on the deviation from the required temperature drop.

In this way materials of all types may be subjected to a sensing means and controlled, following the same basic procedure.

Referring now to the drawings and particularly to FIGS. 1 and 2 thereof, there is illustrated a dryer 10 embodying means in accordance with the present invention for accurately controlling the final moisture content of material being dried in the dryer. The dryer 10 includes a housing or enclosure 12 including a top wall 14 and opposing side walls 16 and 18. Drying is accomplished at least in two zones. To this end the dryer is divided into first and second sections S and S by a wall 29 extending transversely of the housing.

Each section of the dryer is sub-divided into several compartments including a drying chamber 20 extending lengthwise of the dryer and which, as illustrated in FIG. 2, is defined by a vertical partition 22 spaced from and parallel to the side wall 18 and horizontal partion 24 spaced above the surface on which the dryer is supported.

A main conveyor 30 is mounted lengthwise of the dryer actuatable by drive means including a variable speed motor 15 for movement in an endless path in the drying chamber 20. As illustrated, the upper run of the conveyor 30 is disposed in the drying chamber 20 and the lower run below the horizontal partition 24 (see FIG. 2). Conventional feed means including a spiked apron conveyor 34 and a delivery roll 36 are provided adjacent one end of the housing to continuously supply and deposit material to be dried in a layer on the upper run of the conveyor for transport through the drying chamber 20. The dryer may also be fed by an oscillating feed, roller extruder, or granulator for granular materials or preformed chemicals.

A dry-ing medium conditioning chamber 26 extends lengthwise of the dryer to one side of the drying chamber 20 as illustrated. A plurality of motor operated fans 32 spaced along the length of the dryer in the conditioning chamber 26 is provided to circulate drying medium in the direction shown in FIG. 2 downwardly over the upper run of the conveyor and upwardly through the conditioning chamber 26, the partition 22 having openings 31 therein to facilitate such circulation. Conditioning means is provided for cont-rolling the condition of the drying medium comprising, in the present instance, a plurality of heating coils 40 in the conditioning chamber in the path of flow of the drying medium and inlet and outlet conduit-s 42 and 44 for circulating a heating medium such as steam thereth-rough, the inlet conduit having valve means 46 for selectively controlling the rate of steam flow through the heating coils. In place of, or in addition to the heating coils shown, gas burners, electric heaters, or any other heating means may be used.

Drying medium may be exhausted from the drying chamber through an exhaust duct 50 at one end of the housing adjacent the discharge end of the conveyor and which as illustrated, communicates with the interior of the housing at a point below the upper run of the conveyor. An exhaust damper 52 is mounted in the exhaust duct for selectively controlling the amount of drying medium discharged therethrough.

As noted above, since there is usually very little variation in the critical moisture point, due to variations in initial moisture in a given quantity of material, it is possible to calculate with some degree of accuracy the drying conditions necessary to terminate the constant rate drying period P at a predetermined location in the dryer. Thus, for a quantity of material having a predetermined initial moisture content, the drying conditions in the first drying section S may be determined and maintained uniform to provide for termination of the constant rate drying period P at a predetermined relatively fixed location in the first section, for example at a line L Sensing and control means is provided to maintain the temperature of the drying medium circulated through the material at a predetermined uniform level. To this end, a thermal bulb 60 for sensing the temperature of the drying medium delivered to the .bed of material is provided in the drying chamber 20 above the upper run of the conveyor. The bulb 60 is connected to a recording controller 62 which in turn is operatively connected through lines 63 to valve 46 controlling flow of steam through the heating coils 40 in the first drying section S By this arrangement, the amount of heat added to the circulating drying medium may be selectively controlled to provide drying of the material under substantially uniform temperature conditions.

In accordance with the present invention, means is provided for continuously drying material in a manner to compensate for variations in the initial moisture content of some portions of the material being processed. To this end, means is provided for selectively varying the drying conditions, in the present instance in the second section S of the dryer in response to variation-s in the moisture content of the material from a predetermined moisture content. Such variations are determined by comparing the temperature of the drying medium after. passage through the material to the temperature represented by the theoretical line D,,. To acomplish this,

sensing means in the present instance, a thermal bulb 70 is provided below the upper run of the conveyor downstream of the line Lc which measures the temperature of the drying medium after passage through the bed of material on the conveyor at a point designated the scanning point S A sensing means, in the present instance a similar bulb 72 is provided to measure the temperature of the drying medium prior to passage through the bed. The bulb 70 is connected to a temperature transmitter 76 which in turn is connected to a recording controller 77, the bulb 72 also being operatively connected to the controller 77.

The controller 77 is adapted to receive corrective signals from the bulb 70 if the temperature of the drying medium at the scanning point S varies from the line D,, and to effect changes in drying conditions in the second section S of the dryer to compensate for the measured deviation. To this end the controller 77 is connected through leads 80 to variable speed motors 8 1 driving the fans 32 in the second section S of the dryer to selectively control actuation thereof and accordingly regulate the velocity of the drying medium circulated. The controller 77 also operates a slave unit 84 through leads 82 for actuating the damper 52 to selectively control the amount of drying medium vented from the dryer whereby the humidity of the drying medium in the second section S of the dryer may be varied. Leads 86 connect the controller 77 to the valve 46 (see FIG. 4) controlling fluid flow through the heating coils 40 for controlling the temperature in the entering drying medium in the second section S of the drying chamber.

In addition to or in lieu of controlling the humidity in the second drying section S by means of the exhaust damper 52, the humidity may be controlled by means including an inlet duct 200 in the side wall of the housing which mounts therein a de-humidifying unit 202 for removing moisture from outside air introduced through the inlet duct. This arrangement is especially desirable for example, when drying material such as gelatin, for instance, at temperatures such as 90l00 P. (see FIG. 2). The de-humidi-fying -unit 202 is connected through leads 204 to the controller 77.

By this arrangement the bulb 70 continuously senses the dry bulb temperature of the drying medium at the scanning point S and relays it to the transmitter 76. If the temperature transmitted does not fall on the line D,,, the transmitter signals the controller 77 whereby a correction is effected in the drying conditions in the second section S of the dryer. For example, if the temperature of the drying medium at the scanning point S falls above or below the line D as measured by the bulb 70, this indicates that this portion of the material is too dry or too wet and accordingly the recording controller 77 is signalled to effect a change in the drying conditions in the second section S to vary the drying rate and/or the condition of the drying medium to compensate for the condition of the material. The drying rate may be varied by increasing or decreasing the rate of circulation of the drying medium through leads 80 to the fan motors 81. The condition of the drying medium may be varied by raising or lowering the temperature of the drying medium entering the chamber, 'for example by the coil control valve 46 or by varying the humidity, for example by controlling the amount of drying medium exhausted by regulating the damper 52 through leads 82 and slave unit 84 and/or by regulating the de-humidifying unit 202 through leads 204 connecting the same to the controller 77.

It is noted that the controller 77 may be arranged to actuate only one of the elements determining the drying conditions in the second section S or may be arranged to actuate all of the elements either sequentially or simultaneously upon a corrective signal from the transmitter 76. Preferably the adjustment of the fan motors 81 or slave unit 84 to change velocity of the drying medium or position of the damper 52 is effected incrementally or in small steps and continues until the corrective signal from the transmitter 76 to the controller 77 is arrested, whereas the temperature adjustment to vary the temperature of the entering medium is eflected by operating the valve 46 continuously until the desired temperature correction is realized as sensed by the bulb 72.

Consider now the operation of the dryer when processing a continuous run of fibrous material having a given initial moisture content for example. The conveyor speed and fiber feed means 34 and 36 are set relative to one another to deposit a quantity of material of a given thickness on the conveyor 30 and the temperature and rate of circulation of the drying medium are determined, either by calculation or test or both, and set to effect termination of constant rate drying period R, at approximately the line L0 in the first section S of the dryer. In the present instance, the drying conditions in the first section S of the dryer are maintained at the predetermined fixed setting by the controller 62 which effects operation of the control valve 46 (see FIG. 4) for the heating coils in response to temperature variations in the drying medium sensed by the bulb 60 thereby to maintain the temperature of the drying medium uniform. The temperature, flow rate and humidity are determined and set initially in the second drying section S so that if all of the material being processed has a predetermined uniform initial moisture content, the temperature drop of the drying medium follows the line D whereby at a given time T in the dryer, the material advanced through the dryer is characterized by the desired moisture content. The temperature controller 77 in the second section S is initially set at app oximately the same temperature as the controller 62 in the first section S However, if the temperature of the drying medium at the scaning point S registers below the line D for example, on the line D as measured by the bulb 70 this indicates that this portion of the material on the convey-or in the first drying section S is too wet and that it is necessary to change the drying conditions in the second section S to remove the excess moisture. Accordingly the sensing bulb 70 signals the recording controller 77 to increase the temperature and/or increase the velocity of the drying medium and/ or decrease the humidity thereby to increase the rate of drying in the second drying section S As noted above, regulation of the velocity of the drying medium or the amount the exhaust damper 52 is opened is an incremental adjustment to correct the temperature deviation and the temperature is increased continuously until the desired temperature correction is realized as measured by bulb 72 to correct deviation of the actual and theoretical temperatures at the scanning point S and insure the additional necessary drying.

If the drying medium temperature rise is greater than the theoretical value at the scaning point S this indicates that the material is too dry. Accordingly, the controller 77 eifects a decrease in the velocity or temperature of the drying medium and decreases the amount of opening in the exhaust damper 52 to increase humidity to change the drying condition in the second drying section to decrease the rate of drying.

It is noted that if the initial moisture content of all of the material is uniform, the controller 77 does not receive corrective signalsfrom the bulb 70 at the scanning .point S and drying conditions in the second section S remain constant.

Deviations in the temperature from the line D sensed at the scanning point S are due either to variations in the initial free moisture content or to variations in the hygroscopic moisture content of some portions of the material. If the variation is in the free moisture content, the critical point shifts to the left or right of the point C. In either event, there is a temperature deviation from the line D,, at the scanning point, and correction is effected in the manner noted above. If the variation is in the hygroscopic moisture and the temperature drop deviates from a curve as shown in FIG. 7 (designated D,,), then the condition is sense at the scanning point S and a correction is also signalled.

Thus it may be seen that the present invention provides a method and apparatus for drying material to a uniform condition wherein variations in moisture content of some portions are compensated for irrespective of whether the variations are in the free or the hygroscopic moisture content.

There is illustrated in FIGS. 3 and 3a another embodiment of dryer in accordance with the present invention. The dryer 100 comprises an elongated enclosed housing 112 consisting of a top wall 114 and opposing side walls 116 and 118. A pair of transversely extending longitudinally spaced apart partitions 119 and 121 divide the dryer into a first drying section S a second drying section S and a transfer section 8,, intermediate the first and second drying sections.

Each of the drying sections is subdivided into several compartments or zones including a drying chamber 120 extending lengthwise of the dryer and a drying medium conditioning chamber 126 also extending lengthwise of the dryer at one side of the drying chamber 120, the chambers being separated by a partition 122 having openings 131 to facilitate circulation of drying medium therebetween. A plurality of motor operated fans 132 are mounted lengthwise of the dryer in the conditioning chamher 126 of each of the sections to circulate drying medium in the direction shown in FIGS..3a. The dryer includes means for controlling the condition of the drying medium including heating means in the conditioning chamber 126 comprising a plurality of heating coils 140 and inlet and outlet conduits 142 and 144 for circulating a heating medium such as steam through the coils, the inlet conduit 142 having valve means 146 for selectively controlling the rate of steam flow through the heating coils. In place of or in addition to the heating coils shown, gas burners, electric heaters, or any other heating means may be used. An exhaust duct 150 communieating with the drying chamber 120 having a damper control 152 is provided for selectively controlling the humidity of the drying medium in the drying chamber.

In the present instance each dryer section is provided with a conventional conveyor designated 13%, and 1.30 respectively. Accordingly, material to be processed is deposited on the upper run of the conveyor 130 which conveys the material through the first drying section S to the transfer section S where as illustrated, the material is discharged from the first conveyor 130 and deposited on the upper run of the second conveyor 130 in the second drying section S The principle of operation to compensate for variations in the initial moisture content of some portions of the material being dried is basically the same .as that set forth in connection with the first embodiment described above, Thus for material having a given initial moisture content, the conditions in the first drying section S are controlled in a manner to terminate the constant rate drying period P at approximately the line L0 To this end a thermal bulb 160 is provided in the drying chamber of the first section S to continuously sense the temperature of the drying medium circulated through the drying chamber. The thermal bulb 160 is operatively connected to a temperature controller 162 which in turn is operatively connected through the lines 163 to valve 146 (see FIG. 5) controlling flow of steam through the heating coils 140 in the first dryer section S By this arrangement, the speed of the conveyor and the rate of flow of the drying medium may be calculated and set to provide for termination of the constant rate drying period at the line L0 in the first drying section S the bulb 164] and controller 162 serving to maintain the temperature of the drying medium circulated at a predetermined uniform level in the first drying section.

In the present instance, means is provided for selectively varying the drying conditions in the second drying section S to compensate for variations in the moisture content in some portions of the material being processed. To this end, sensing means in the form of a thermal bulb 170 is provided below the upper run of the conveyor in the first drying section S downstream of the line L61 which measures the temperature of the drying medium at a point designated the scanning point S and another thermal bulb 172 is provided downstream of the scanning point S in the second drying section S above the upper run of the conveyor to measure the temperature of the drying medium delivered to the bed of material on the conveyor in the second dryer section S The bulb 170 is connected to a temperature transmitter 176 which in turn is connected to a recording controller 177, the bulb 172 also being operatively connected to the controller 177.

By this arrangement, the temperature of the drying 7 medium sensed at the scanning point is transmitted to the temperature transmitter 176 and if the temperature varies from the line D,,, the transmitter signals the controller 177 to effect change in the drying conditions in the second drying section. To accomplish this, the controller 177 is connected through leads 180 tovariable speed fan meters 181 in the second section S of the dryer to regulate the velocity of the drying medium circulated and is also connected through leads 182 to a slave unit 184 actuating the damper 152 thereby to vary the amount of drying medium vented from the second section of the dryer whereby the humidity of the drying medium may be cont-rolled. In addition to, or in lieu of the damper 152 an inlet duct 300 having a de-humidifying unit 302 therein and leads 304 connecting the de-humidifying unit 302 to the controller may be provided to control humidity of the drying medium. Further the controller 177 is adapted to operate valve 146 to control flow of steam through the coils for controlling temperature of the drying medium. The controller 177 is also connected to the variable speed motor M for the conveyor 130 whereby the rate of movement of the mate rial through the second section may be varied.

Consider now the operation of the dryer 100 for drying material having a predetermined initial moisture content as determined by a given sampling of'the material. Based on this sampling, the drying conditions are calculated for the first dryer section S so that the constant rate drying period terminates at approximately the line L0 It is noted that the bulb and controller 162 cooperate to maintain a substantially uniform temperature of the drying medium circulated through the material in the first dryer section S The temperature controller 177 in the second dryer section S is initially set at a temperature approximately the same as, or slightly lower than, the controller 162 in the first drying section S If a portion of the material has a moisture content greater or less than the predetermined amount, a varia- 9 tion is reflected at the scanning point S whereby tern perature of the drying medium will register above or below the line D,, The temperature deviation is sensed by the bulb 170 and the temperature transmitter 176 which transmits a corrective signal to the controller 177. The controller effects a change in the drying conditions in the second dryer section S to compensate for the deviation in the manner similar to that described in connection with the first embodiment. The controller 177 may eifect a change in combination or solely in the flow rate of the drying medium through leads 180 connecting the controller to the fans 181, in the temperature of the drying medium through leads 186 connecting controller 177 to the valve 146 for the coils 140 and/or the humidity of the drying medium through leads 182 connecting controller 177 to the slave unit 184 for the damper 152 in the second section S If desired, humidity of the drying medium may be controlled through the de-hum-idifying unit 302. In addition according to this embodiment of the invention, the rate of movement of the material through the second section S may be varied to selectively decrease or increase the time of the material in the dryer through leads connecting controller 177 to the motor M for conveyor 130 From the foregoing, it may be seen that the present invention provides a highly effective method and apparatus for continuously drying material which may have a variable moisture content so that all of the material when dried is characterized by substantially uniform moisture content. As noted above, the cycle for drying material follows a predetermined pattern comprising a constant rate drying period and a falling rate drying period, or a falling rate drying period, and the present invention utilizes this principle to effect drying of material to a substantially uniform moisture content by sensing at a point along the path designated the scanning point, the dry bulb temperature of the drying medium after it is passed through the material which in effect is sensing the moisture of the material at that point. Then, depending on whether the temperature of the drying medium sensed at this point varies from a predetermined temperature, the sensing means eifects a correction in drying conditions to compensate for any variations whereby the dried material is characterized by substantially uniform moisture content. Thus it can be appreciated by simply sensing dry bulb temperatures, the highly desired result of obtaining a uniform moisture content of all the material being processed is obtained.

While particular embodiments of the apparatus of the present invention and a method for drying material have been illustrated and described herein, it is of course to be understood that changes and modifications may be made therein within the scope of the following claims.

I claim:

1. A method of continuously drying material to a desired condition consisting of the steps of conveying material along a predetermined path through at least first and second zones, controlling the drying conditions by means including circulating aconditioned drying medium through the material to dry the same, sensing the temperature of the drying medium after it has passed through the material at least at one given point in one of said zones and varying the drying .conditions in at least one of said zones in response to deviations of the temperature of the drying medium sensed at said point from a predetermined temperature at said point.

2. A method as claimed in claim 1 wherein the condition of the drying medium in said first zone is fixed.

3. A method as claimed in claim 1 wherein the drying conditions are varied by selectively varying the temperature of the drying medium circulated through said material in said second zone to compensate for deviations of the temperature of the drying medium sensed at said point from said predetermined temperature.

4. A method as claimed in claim 1 wherein the drying conditions are varied in said second zone by selectively regulating the humidity of the drying medium in said second zone to compensate for deviations of the temperature of the drying medium sensed at said point from said predetermined temperature.

5. A method as claimed in claim 1 wherein the drying conditions are varied in said second zone by selectively regulating the condition of the fresh air entering said second zone to compensate for deviations of the temperature of the drying medium sensed at said point from said predetermined temperature.

6. A method as claimed in claim 1 wherein the drying conditions are varied by selectively varying the rate of circulation of the drying medium through the material in said second zone to compensate for deviations of the temperature of the drying medium sensed at said point from said predetermined temperature.

7. A method as claimed in claim 1 wherein the drying conditions are varied by selectively varying the rate of movement of said material along said path in said second zone to compensate for deviations of the temperature of the drying medium sensed at said point from said predetermined temperature.

8. A method as claimed in claim 1 wherein the drying conditions are varied in said second zone by simultaneously varying the temperature, humidity, and velocity of the drying medium in said second zone and the rate of movement of the material along said path inv said second zone and condition of fresh air entering said second zone to compensate for deviations of the temperature of the drying medium sensed at said point from said predetermined temperature.

9. A method as claimed in claim 1 wherein the temperature sensed at said given point is the dry bulb temperature of the drying medium after it has passed through the bed of material to compensate for deviations of the temperature of the drying medium sensed at said point from said predetermined temperature.

10. A method of continuously drying material to a desired condition consisting of the steps of conveying material along a predetermined path through at least two zones, controlling the drying conditions by means including circulating a heated drying medium heated to a predetermined temperature through the material so that at a given point along said path the temperature of said drying medium after it has passed through said material is lowered to a predetermined temperature, sensing the temperature of the drying medium after it has passed through the material at least at said given point and varying the drying conditions in at least one of said zones in response to deviations of the temperature of the drying medium sensed at said point from a predetermined temperature at said point.

11. A method of continuously drying material to a desired condition consisting of the steps of conveying material along a predetermined path through at least two zones, maintaining fixed drying conditions in one of said zones, controlling the drying conditions in the other of said zones by means including circulating a heated drying medium through the material to dry the same, sensing the temperature of the drying medium after it passes through the material at least at one given point in the other zone and varying the drying conditions in said other zone in response to deviations of temperature the material is constant and a falling rate drying period wherein the theoretical temperature drop of the drying medium across the material gradually decreases, the juncture of the constant rate and falling rate being the theoretical critical point, consisting of the steps of continuously conveying the material through first and second treatment zones, maintaining drying conditions in said zones to effect drying of said material throughout said constant rate drying period and into said falling rate drying period, sensing at a scanning point in the falling rate dry-ing period the actual temperature drop of the drying medium relative to a theoretical temperature drop at said scanning point and varying the drying conditions in at least said second treatment zone when the actual temperature drop at said scanning point deviates from said theoretical temperature drop at said scanning point whereby variations in the moisture content in some portions of the material are compensated for and all of the dried material is characterized by a substantially uniform moisture content.

13. A method as claimed in claim 12 wherein a su stantial portion of the material has a varying moisture content.

' 14. A method as claimed in claim 12 wherein the drying conditions are varied by selectively varying the temperature of the drying medium circulated through said material in said second zone to compensate for deviations of the temperature of the drying medium sensed at said scanning point from said predetermined temperature.

15. A method as claimed in claim 12 wherein the drying conditions are varied in said second zone by selectively regulating the humidity of the drying medium in said second zone to compensate for deviations of the temperature of the drying medium sensed at said scanning point from said predetermined temperature.

16. A method as claimed in claim 12 wherein the drying conditions are variedby selectively varying the rate of circulation of the drying medium through the material in said second zone to compensate for deviations of the temperature of the drying medium sensed at said scanning point from said predetermined temperature.

17. A method as claimed in claim 12 wherein the drying conditions are varied by selectively varying the rate of movement of said material along said path in said second zone to compensate for deviations of the temperature of the drying medium sensed at said scanning point from said predetermined temperature.

18. A method as claimed in claim 12 wherein the drying conditions are varied in said second zone by selectively varying the condition of the fresh air entering said second zone to compensate for deviations of the temperature'of the drying medium sensed at said scanning point from said predetermined temperature.

19. A method as claimed in claim 12 wherein the drying conditions are varied in said second zone by simultaneously varying the temperature, humidity, and velocity of the drying medium in said second zone and the rate of movement of the material along said path in said second Zone and condition of fresh air entering said second zone to compensate for deviations of the temperature of the drying medium sensed at said scanning point from said predetermined temperature.

20. Apparatus for continuously drying material to a desired condition comprising an elongated, generally enclosed housing partitioned at a point along its length to divide the interior into at least first and second treatment Zones, each of said treatment zones including a drying chamber, means for conveying the material along a predetermined path through the drying chamber of each of said zones, drying medium conditioning means including means for circulating a drying medium through the material to dry the same, sensing means for sensing the temperature of the drying medium after it passes through the material being dried at least at one given point in the terminal portion of the first zone, and means operatively connecting said sensing means and conditioning means whereby the drying conditions in at least one of said zones may be varied in response to deviations of the temperature of the drying medium sensed at said' point from a predetermined temperature thereby to selectively vary the drying rate.

21. Apparatus as claimed in claim 20 wherein said drying medium conditioning means includes heating means for heating saiddrying medium circulated through said drying chamber and control means for selectively regulating the amount of heat added to the drying medium and wherein said sensing means is operatively connected to said control means whereby the temperature of the drying medium may be varied in response to deviations of the said dry-ing medium temperature sensed by said sensing means from a predetermined temperature.

22. Apparatus as claimed in claim 20 wherein said means for circulating drying medium comprises a fan and fan drive means operably connected to said sensing means whereby the rate of circulation of the drying medium through said drying chamber may be varied in response to deviations of the drying medium temperature sensed by sensing means from a predetermined temperature.

23. Apparatus as claimed in claim 20 wherein said drying medium conditioning means includes means in said second zone for controlling the humidity of the drying medium circulated therethrough comprising an exhaust duct communicating at one end with the housing and open at its opposite end to the atmosphere, a damper in said exhaust duct and damper control means for selectively varying the position of the damper and wherein said sensing means is connected to said damper control means thereby to control the amount of drying medium discharged through said duct in response to deviations of said drying medium temperature sensed by said sensing means from a predetermined temperature.

24. Apparatus as claimed in claim 20 wherein said drying medium conditioning means includes duct means for introducing fresh air in said second zone and means in said duct for selectively varying the humidity of said air.

25. Apparatus as claimed in claim 20 wherein said means for conveying material along a predetermined path includes first and second conveyors mounted respectively in said first and second treatment zones, first and second conveyor drive means for said first and second conveyor means, means operably connecting said sensing means to said second conveyor drive means whereby the rate of movement of material through said second treatment zone may be selectively varied in response to deviations of the temperature of the drying medium sensed by said sensing means at said point from a predetermined temperature.

References Cited by the Examiner UNITED STATES PATENTS 2,107,275 2/1938 Anderson et al 343l X 2,309,993 2/ 1943 Skagerberg 34-48 X 2,391,195 12/1945 Ross et a1 3448 X 2,799,096 7/1957 Scott 34-13 2,874,482 2/1959 Haltmeir 3431 ALDEN D. STEWART, Primary Examiner.

WILLIAM F. ODEA, Examiner.

J. J. CAMBY, Assistant Examiner. 

1. A METHOD OF CONTINUOUSLY DRYING MATERIAL TO A DESIRED CONDITION CONSISTING OF THE STEPS OF CONVEYING MATERIAL ALONG A PREDETERMINED PATH THROUGH AT LEAST FIRST AND SECOND ZONES, CONTROLLING THE DRYING CONDITIONS BY MEANS INCLUDING CIRCULATING A CONDITIONED DRYING MEDIUM THROUGH THE MATERIAL TO DRY THE SAME, SENSING THE TEMPERATURE OF THE DRYING MEDIUM AFTER IT HAS PASSED THROUGH THE MATERIAL AT LEAST AT ONE GIVEN POINT IN ONE OF SAID ZONES AND VARYING THE DRYING CONDITIONS IN AT LEAST ONE OF SAID ZONES IN RESPONSE TO DEVIATIONS OF THE TEMPERATURE OF THE DRYING MEDIUM SENSED AT SAID POINT FROM A PREDETERMINED TEMPERATURE AT SAID POINT. 